Optical ranging systems can collect distance data of one or more objects in a scene. Distance data can include, for example, the distance between the one or more objects and the optical ranging system. Optical ranging systems typically contain an illumination source and a plurality of pixels. The pixels can be operable to collect incident light and can generate distance data. The pixels can be, for example, time-of-flight pixels. Generally, the pixels, including associated circuity, can collect incident light and generate electrical charge carriers over a particular integration time, with a particular sensitivity, and with a particular amplification.
Optical ranging systems with such pixels typically direct light from the illumination source onto the object(s) in the scene. Some of this light reflects from the object(s) and is directed to the plurality of pixels. Only a portion of this light, however, is incident on the pixels. Further, that portion may vary from object to object as the portion of light reflected and incident on the pixels is dependent on the reflectivity and distance of the object(s) from the optical ranging system. For example, objects that are closer to the optical ranging system can reflect more light back to the plurality of pixels than objects that are further away.
In order to generate distance data, the pixels must capture incident light reflected from the object(s), the light generates charge carriers, and the charge carriers are converted into a signal that is subsequently correlated with distance data of the object(s). In some instances, the pixels may not capture enough light to generate sufficient charge carriers to generate accurate distance data; that is, the signal-to-noise ratio may be too low to be useful. In such instances, the pixels are under-exposed. In other instances, the pixels may capture too much light, thereby generating too many charge carriers that subsequently saturate the pixel. In such instances, the pixels are over-exposed and the signal is not capable of generating accurate distance data. Between these two extremes, however, the pixels may capture sufficient light to generate accurate distance data; that is, the signal-to-noise ratio may be high enough to produce accurate distance data. In such instances, the pixels are adequately exposed.
The dynamic range of incident light required to generate adequately exposed pixels is customizable since the amount of light reflected from the object(s) and incident on the pixels can vary significantly. Consequently, various techniques have been developed to alter the dynamic range of the pixels. In some instances, for example, the integration time can be dependent on the intensity of light incident on the pixels, and can be increased to collect more light and generate more charge carriers, or decreased to collect less light and generate fewer charge carriers. Similarly, in some instances, pixel sensitivity can be altered. For example, in some time-of-flight pixels, the sensitive area of each pixel can be increased to collect more light and generate more charge carriers, or decreased to collect less light and generate fewer charge carriers. While effective, altering integration time, sensitivity, and/or amplification can consume significant resources. It can be particularly time consuming, for example, and therefore is a challenge to implement in optical ranging systems used in real-time or near real-time applications.